Radio transmitting system



Patented Feb. 22, 1949 RADIO TRANSMITTING SYSTEM Walter H. Grosselfinger, Manhasset, N. Y., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application September 28, 1944, Serial No. 556,111

1 Claim.

This invention relates to radio transmitting systems and, more particularly, to radio transmitting systems employing frequency modulation.

An object of the invetion is to, provide radio transmitting systems with improved frequency modulation circuits.

Another object is to provide for increased frequency modulation of piezoelectric; controlled oscillators.

It is also an object of the invention to provide radio transmitting systems with improved frequency modulation circuits for causing the frequency of radiated wave energy to be varied between two difierent values.

An additional object is to provide a frequency modulation circuit with improved means for increasing the range of the frequency changes produced during frequency modulation opera tions.

Another object is to provide radio transmitting systems with improved frequency modulation circuits for shifting the frequency of the radio carrier wave energy from one value to another in accordance with signals.

Another object is to provide a radio transmitting system with an improved electronic frequency shift keying circuit.

A further object 'is to improve the performance of keying circuits in radio telegraph transmitting systems by reducing keying transients.

These and other objects are attained by employing an improved electronic keying circuit for varying the frequency of wave energy generated by a high frequency thermionic oscillator. This keying circuit comprises a piezoelectric crystal connected in series with the grid of the oscillator for determining the frequency of the wave energy generated by the oscillator. An electronic switch, in the form of a double-diode rectifier, and a small capacitance are connected in series and are also connected in parallel across the crystal. When the double-diode rectifier is not conductive this circuit will be open with the result that the oscillator will generate wave energy of a particular frequency, such as the marking frequency. On. the other hand, when positive current is applied to the anodes of the double-diode rectifien as by the. action of a keying relay. the re inertube will. become. Qimductive. This, in e -est. clos s. the Parallel. cir u connec he condenser across the crystal thereby causing the frequency of the wave. energy generated by the thermionic oscillator to be reduced toa lower value; namely, the spacing frequency. An inductance is connected in series with the crystal and the grid of the oscillator for the, purpose of expanding the frequency change produced in keying so that there will be a relatively wide, frequency separation between the marking and spacing frequencies. A wave shaping filter, such as aresistance-capacitance network, is connected in the current supply circuit of the double-diode rectifier for effecting a more gradual slope in the conducting characteristic of this tube in order to reduce keying transients which would tend to be created if the condenser should be abruptly connected across the crystal. The principles of the invention are not limited solely to the transmission of telegraph signals but are also applicable to the transmission of audio frequency signals.

These and other features of the invention are more fully explained in connection with the following detailed description of the drawing, in which:

Fig. 1 illu-strates'a radio telegraph transmitting system provided with, a preferred form of the improved electronic frequency shift keying circuit;

Fig. 2 shows the invention applied to a voice frequency transmitting system{ and Fig. 3 shows the invention applied to a combined telegraph and voice frequency radio transmitting system.

In Fig. 1, a radio transmitting system is shown to be provided with a source of marking and spacing polar telegraph signals represented by a negative battery I, a positive battery 2, and a manually operable key 3. Operation of the key 3 alternatively connects batteries I and 2 to ground 4 through the winding of a polarized keying relay 5. Since relay 5 is polarized, it will operate its armature alternatively to its left and right contacts to connect either negative battery 6 or positive battery 1 to the anodes of a doublediode rectifier 8. This current supply circuit for the double-diode 8 contains a relatively large isolating resistance 9: and a wave shaping filter constituted by a resistance-capacitance network comprisinga resistance Ill and a capacitance H. Incidentally, it should be noted thatthe positive battery I is considerably larger than the, negative change during keying operations.

battery 6. A voltage regulator I 2 is connected to the positive battery I for maintaining a constant voltage supply for the double-diode 8.

The rectifier 8 constitutes an electronic switch for shifting the frequency of the wave energy generated by a high frequency thermionic oscillator I3 from the marking frequency to the spacing frequency and vice versa without changing the amplitude of this wave energy. Since the frequency of the waveenergy enerated by oscillator I3 is controlled by a piezoelectric crystal I4 connected to the grid of oscillator I3, the frequency shift is effected by causing the rectifier tube 8 to switch a small variable air condenser I5 in parallel with crystal I4 whenever it is desired to transmit wave energy having the spacing frequency.

In order to expand the frequency separation between the marking and spacing frequencies, a small inductance I5 is connected in the grid circult of oscillator I3 in series with crystal I4. It should be noted that the bridge circuit containing the double-diode 8 and the condenser I5 is connected across both the crystal I4 and the inductance I6. This permits the use of a larger capacity for condenser I5 than would otherwise be possible. Bymeans of this combination, it is possible to obtain a relatively wide frequency For example, in a preferred embodiment of the invention, the keying circuit produces a frequency separation of 850 cycles between the marking and spacing frequencies. The inductance I6 also increases the stability of the circuit and improves its operation, particularly when the condenser I5 is bridged across the crystal I4 and the inductance I 6.

The use of the electronic switch 8 instead of an electromagnetic relay for switching the condenser I5 across the crystal I4 and its associated inductance I6 reduces the occurrence of keying transients that might otherwise be produced by abruptly connecting condenser I5 across crystal I 4 and inductance I8. Keying transients are further reduced by means of the wave shaping filter, constituted by resistance In and capacitance I I, connected in the current supply circuit of the double-diode rectifier 8. This is due to the fact that the wave shaping filter effects a more gradual change in the conducting characteristic of the double-diode 8 thereby avoiding the creation of steep wave fronts.

In operating the system of Fig. 1, marking and spacing polar telegraph signals are originated by the operation of key 3. When a marking signal is to be transmitted, key 3 is operated to the position shown in Fig. 1 to connect negative current from battery I to the winding of the keying relay 5. This causes relay 5 to operate its armature to its marking contact thereby supplying negative current from battery 6 to the anodes of the double-diode 8 to prevent it from being conductive. The parallel circuit containing condenser I5 will now be open and the crystal controlled oscillator I3 will now produce wave energy having the marking frequency. This wave energy is amplified by an amplifier IT and is further amplified by a power amplifier I8 which has its outnut'connected to the transmitting antenna 20.

When it is desired to transmit a spacing signal, key 3 is operated to connect positive current from battery 2 to the winding of the keying relay 5. Relay 5 now operates its armature to its spacing contact to connect positive current from battery 2| battery I to the anodes of the double-diode 8. The rectifier 8 thus becomes conductive thereby connecting condenser I5 in parallel with crystal I4 and inductance I6. Thus causes the frequency of the wave energy produced by the crystal controlled oscillator I3 to be lowered to the spacing frequency. v

In some applications of the invention, it is desirable to be able to switch the marking and spacing frequencies quickly from one set of Values to another. This can be accomplished by employing two sets of the crystal-inductance-condenser combination with a switch for alternatively connecting either set into the keying circuit. Such a switch is constituted by the switching relay which is energized by current from whenever the manually operable switch 22 is closed. When switch 22 is open, as

is shown in Fig. 1, relay 20 is not energized and its top and bottom armatures are both in engagement with their outer contacts. Under this condition, the system will transmit wave energy having alternative marking and spacing frequency values which are determined by the crystal I I-inductance Iii-condenser I5 combination in the manner described above.

In order to transmit wave energy having different marking and spacing frequencies, a different crystal 24 and a different series connected inductance 26 are connected to the lower inner contact of relay 20 in the manner shown in Fig. 1 and a different small variable air condenser 25 is connected to the upper inner contact of relay 20. When the switch 22 is closed. current from battery 2 I will energize relay 20 which will move its top and bottom armatures into engagement with their inner contacts. This will switch the crystal I i-inductance Iii-condenser I5 combination out of the keying circuit and will connect the crystal 24-inductance ZB-condenser 25 combination into the keying circuit. The wave energy that will now be produced by oscillator I3 will have either a marking frequency determined by crystal 24 and inductance 26 or a spacing frequency determined by condenser 25, crystal 24, and inductance 26. Thus, by these means. the system shown in Fig. 1 can be switched readily from the transmission of wave energy having one set of marking and spacing frequencies to the transmission of wave energy having a different set of marking and spacing frequencies. It is to be understood that the invention is not limited to the specific source of telegraph signals shown in the drawing as various other means of supplying telegraph signals may be employed, such as a teletypewriter.

Fig. 2 illustrates the manner in which the invention is applied to an audio frequency radio transmitting system and shows a radio transmitting system somewhat similar to that shown in Fig. 1 but provided with a source 20!) of audio frequency signals of varying amplitude instead of a source of telegraph signals. The source 205 of audio frequency signals may be of any suitable design, such as a handset and its associated circuit for converting speech waves into electric waves of varying amplitude. Audio frequency signals from the source 200 are supp ied to the primary winding of a transformer 23]. Transformer 23I has its secondary winding connected in series with the current supply circuit of a double-diode rectifier 208 through an inductance and a potentiometer 235. This current supply circuit includes a positive battery 20'! and a voltage regulator 2l2. Rectifier tube 29 8; and a s mall 'variable air condenser M are, in turn, eenn et dgin parallel ith. a. cir en inins a crystal 2M connected in series with a small inductance 2H5. Crystal 2M and its series connected inductance 216' are connected to the grid of a frequency thermionjg oscillator-M3.

In operating the system shown in; Fig. 2, potentiometer fiitiisso adiustedthat, When there is an absence of'audio frequency signals, rectifier iiitvv llv be. partially conduct and wi l: be: ope

ating at a point approximately at the mid-point of its conducting characteristic. When audio frequency signals are present, they are superimposed upon that portion of the direct current from battery 20'! which passes over the potentiometer 235 and through the secondary winding of transformer 23L This causes a fluctuating direct current to be supplied to the anodes of the double-diode rectifier 238 so that it alternatively becomes moreconductive and less conductive in accordance with the variations in the amplitude of the audio frequency signals. Since the charge on condenser 2l5 varies in proportion to the conductivity of double-diode 208, the effect of placing condenser 3L5 in parallel with crystal 2M and inductance 2H5 will vary in accordance with the variations in the conductivity of the rectifier tube 208. In other words, the wave energy produced by the crystal controlled oscillator 2l3 will vary in frequency in a manner corresponding to the variations in the amplitude of the audio frequency signals. The wave energy from oscillator H3 is amplified by an amplifier 2|! and is also further amplified by a power amplifier 2I8. The output of power amplifier 218 is connected to a transmitting antenna 219 which radiates the frequency modulated wave energy into space for reception by a frequency modulation receiving system.

Fig. 3 illustrates the manner in which the invention is applied to a combined telegraph and voice frequency radio transmitting system for transmitting either telegraph signals or audio frequency signals. The circuit elements of the system shown in Fig. 3 are somewhatsimilar to those shown in Figs. 1 and 2 but include, in addition, a control switch 330. The operation of switch 33! which is shown in the form of a cradle switch, is controlled by the operation of the handset 300, which constitutes a source of audio frequency signals of variable amplitude. When the handset 300 is resting in its cradle 350, the cradle switch 330 will be open as shown in Fig. 3. When the handset 30!! is lifted out of its cradle 360, switch 330 will be closed.

If it is desired to transmit telegraph signals instead of speech signals, key 303 is operated between its contacts to connect either positive current from battery 302 or negative current from battery 3M to the winding of polarized relay 385. Since relay 335 is polarized, positive current in its winding will cause its armature to engage its spacing contact and negative current in its winding will actuate its armature into engagement with its marking contact thereby closing a circuit extending from a small negative battery 333 to potentiometer 335. Potentiometer 335 is connected in the grid circuit of a triode 308 and is so adjusted that triode 308 is normally operating at a point approximately at the mid-point of its conducting characteristic. If battery 333 is connected to the grid circuit of triode 308 by relay 305,1then triode 308 will cease to be conductive and condenser 31-5 will not be connected in parallel across crystal- 314 and its series connected inductance 3| 6. As a result, the crystal controlled oscillator 313 will produce wave energy having the marking frequency. If'battery 333 is not connected to the grid circuit of triode 338, as is the case when positive I 7 332.! is applied to the winding of relay 335' to actuate its armature into engagement with its pac contact, he. triode 3. .8 W be partially conductive to a sufficient extent to connect condenser 3l5'in parallel with crystal 3% and its series connected inductance 3l6. As was explained above in connection with the description of the operation of the system shown in Fig. 1, this will lower the frequency of the wave energy produced by the crystal controlled oscillator 3l3 to the spacing frequency.

On the other hand, if it is desired to transmit speech signals instead of telegraph signals, then the removal of the handset 333 from its cradle 360 will effect the closure of the control switch 333, as was described above, to connect negative current from battery 333 to potentiometer 335 thereby short-circuiting the path extending from battery 333 through the armature of relay 305. The audio frequency signals from the handset 333 are superimposed by transformer 33! upon the negative grid current from battery 333 and the resulting fluctuating direct current causes triode 3&8 to alternatively become more conductive and less conductive in accordance with the variations in the amplitude of the audio frequency signals. These variations in the conductivity of triode 338 will, as was explained above in connection with the description of the operation of the system shown in Fig. 2, cause the frequency of the wave energy now produced by the crystal controlled oscillator 353 to vary in a manner corresponding to the variations in the amplitude of the audio frequency signals.

What is claimed is:

A signal transmitting system comprising in combination a source of telegraph signals including a telegraph relay, a source of audio frequency signals including a telephone handset provided with a cradle switch, an electronic oscillator for generating high frequency electric wave energy,

a piezoelectric crystal for controlling said oscillator for determining the frequency of said wave energy, and modulating means for effecting changes in the frequency of said wave energy from one wavelength to a different wavelength in accordance with variations in said signals, said modulating means including a condenser and an electronic tube connected in series with each other and in parallel with the crystal, said electronic tube having a plurality of electrodes including a grid, a source of grid bias voltage for said tube, and control means for effecting changes in the conductivity of said tube in accordance with variations in said signals, said control means being characterized by having (References on following page) current from battery 7 REFERENCES CITED The following references are of record in the file of this patent:

STATES PATENTS Number Name Date Brackett Dec. 15, 1925 Trouand Apr. 3, 1934 Goddard Apr. 30, 1935 Fichandler May '7, 1935 Koerner Dec. 2, 1941 Number Number OTHER REFERENCES Termans Radio Engineers Handbook of 1943. 

